mruby-bigint: update README.md

The document is written by Google Jules.

mrbgems/mruby-bigint/README.md

@@ -1,7 +1,44 @@
-# Multi-precision Integer extension for mruby
+# mruby-bigint
+
+mruby-bigint is an mrbgem that provides multi-precision integer (BigInt) support for mruby. It allows you to work with integers that are larger than the standard Integer type can handle.
 
 This extension uses fgmp, which is a public domain implementation of a subset of the GNU gmp library by Mark Henderson <markh@wimsey.bc.ca>.
 But it's heavily modified to fit with mruby. You can get the original source code from <https://github.com/deischi/fgmp.git>.
 You can read the original README for fgmp in [README-fgmp.md](README-fgmp.md).
 
 If you want to create your own Multi-precision Integer GEM, see [examples/mrbgems/mruby-YOUR-bigint/TODO-HINT.md](../../examples/mrbgems/mruby-YOUR-bigint/TODO-HINT.md).
+
+## Features
+
+- Basic arithmetic operations: `+`, `-`, `*`, `/`, `%`
+- Power operation: `**`
+- Modular exponentiation
+- Bitwise operations: `&`, `|`, `^`, `<<`, `>>`
+- Comparison: `<=>`
+- Conversion to and from strings: `to_s`, `String#to_i` (with base)
+- Square root
+- Greatest Common Divisor (GCD) (available if `MRB_USE_RATIONAL` is defined)
+
+## Usage
+
+Here are some simple examples of how to use mruby-bigint:
+
+```ruby
+# Creating BigInts
+a = BigInt(12345678901234567890)
+b = "98765432109876543210".to_i(10) # Specify base 10 for string conversion
+
+# Arithmetic operations
+c = a + b
+puts c.to_s # Output: 111111111011111111100
+
+d = a * 2
+puts d.to_s # Output: 24691357802469135780
+
+# Comparison
+puts a <=> b # Output: -1
+```
+
+## fgmp Dependency
+
+mruby-bigint depends on fgmp. For more information about fgmp, please see [README-fgmp.md](README-fgmp.md).